The present invention relates to an aqueous dispersion of a crosslinkable polyurethane obtained by chain extension of a polyurethane prepolymer with a compound containing hydrazone functions having the general formula: 
wherein
Q1 and Q2 is a hydrazone functional group of formula (2), H or R1 
R and R1 in formula (1) is an organic group which has no reactivity towards the hydrazone function,
R2 and R3 is an organic group which has no reactivity towards the hydrazone functions, with the proviso that at least one of the groups Q1 or Q2 in formula (1) is of formula (2) It relates to a process for the preparation of said polyurethane dispersion, to a process for the preparation of a crosslinked film or a coated substrate and to the obtained crosslinked film or coated substrate.
It is well known that the performance and properties of coatings or films of aqueous polyurethane dispersions can be increased by the incorporation of crosslinkable groups in the polymer backbone and crosslinking of said polymers with relevant crosslinkers. For application onto temperature sensitive substrates, such as leather and artificial leather, the low-temperature baking types are of special interest. An overview of low baking type cross-linking for polyurethane dispersions is presented in EP-507407. This application describes further an improved crosslinking system in which carboxylic acid functional polymers are crosslinked by carbodiimide functional oligomers or polymers containing additional reactive functional groups, which contribute to the crosslinking.
An alternative crosslinking system is described in U.S. Pat. No. 4,598,121, which comprises the incorporation of a hydrazide functional chain extender in a polyurethane and crosslinking of the polyurethane with formaldehyde.
Since the use of formaldehyde type crosslinkers are expected to cause environmental problems, other types of compounds which are reactive towards the hydrazide function became of interest as crosslinkers, such as isocyanate functional, ketone functional or epoxide functional compounds. Research has shown that, because of the reactivity of these groups towards the hydrazide function, especially of the isocyanate functional compounds, the potlife of the hydrazide functional polyurethane together with the crosslinkers was limited. Moreover, it appeared that the polyurethane dispersions containing hydrazide functions became yellow in stability tests.
We now developed a new aqueous dispersion of a cross-linkable polyurethane in which a product containing blocked reactive sites is incorporated in the polymer chain. The obtained aqueous polyurethane dispersion combined with a relevant crosslinker is suitable for the preparation of a crosslinked film or of a crosslinked coating onto a substrate.
The present invention provides an aqueous dispersion of a crosslinkable polyurethane obtained by chain extension and/or chain terminating of an isocyanate functional polyurethane prepolymer with a compound which contains hydrazone functions having as the general formula: 
wherein
Q1 and Q2 is a hydrazone functional group of formula (2), H, or R1 
R and R1 in formula (1) is an organic group which has no reactivity towards the hydrazone function,
R2 and R3 in formula (2) is an organic group which has no reactivity towards the hydrazone function,
x has a value from 1 to 10, and is preferably from 1 to 4 with the proviso that at least one of the groups Q1 or Q2 is of formula (2).
In an aqueous solution at pH above 7 the hydrazone function is stable, but at lower pH, or during drying at elevated temperatures the hydrazone function in the polyurethane reacts with water under formation of a ketone and a hydrazide function of formula 
The resulting product which contains hydrazide functions can be crosslinked and/or chain extended by a polyisocyanate, a polyketone or a polyepoxy functional material.
Since the product of formula (1) is used as a chain extender and/or chain terminating group for isocyanate functional polyurethane prepolymers at least one of the groups Q must be a hydrogen. The invention therefore includes the aqueous dispersion of a polyurethane which contains as chain extender or chain terminating group a hydrazone functional compound of formula (1), wherein at least one of the groups groups Q1 or Q2 is H.
It appeared that under aqueous conditions only the NH or NH2 function of the product of formula (1) reacts with a isocyanate functional material under formation of a urea function, while the hydrazone function remains intact. In the presence of only one hydrogen reactive amine in formula (1) the isocyanate functional material is just capped.
More preferably the product of formula (1) contains two or more hydrogen reactive amine functions and it can chain extend and/or crosslink a polyisocyanate functional material to obtain an aqueous dispersion of a polyurethane containing hydrazone functions. Accordingly the present invention relates to an aqueous dispersion of a polyurethane which preferably contains as chain-extender a hydrazone functional product, which contains at least two hydrogen functional amino groups.
Usually the product of general formula (1) which is used as chain extender in the aqueous polyurethane dispersion is prepared by reacting a hydrazide functional compound of formula 
wherein
P1 and P2 is a hydrazide functional group of formula (3) or is H, or is R1,
R and R1, and x are as in formula (1)
with the proviso that at least one of the groups p1 or p2 has formula (3)
with an equimolar amount of a ketone functional compound of formula 
wherein R2 and R3 are as in formula (2).
The hydrazide functional compound is prepared from a polyamine by a method as described in U.S. Pat. No. 4,598,121. The polyamine has the structure conform formula (4) in which the hydrazide functional group P was originally hydrogen. According to this method the polyamine is first reacted with an alkyl-acrylate whereafter the obtained alkyl aminopropionate functional compound is reacted with hydrazine.
The hydrazide functional compound is a hydrazide functional alkylamine, a cycloalkylamine, an arylamine, an alkylpolyamine, a cycloalkylpolyamine, an arylpolyamine, an alkylamino-cycloalkylamine, a polyalkoxyamine, a polyalkoxypolyamine, an aminoalkylamine, an amino-poly(alkylamine).
The preferred hydrazide functional compound can be selected from a hydrazide functional 5-amino-1,3,3-trimethylcyclohexanemethylamine, methylpentanediamine, diaminohexane, 3,3xe2x80x2-dimethyl-4,4xe2x80x2-diaminodiyclo-hexylmethane, 4,4xe2x80x2-diamino-dicyclohexylmethane, a polyoxypropylenediamine with a molecular weight of around 230, 400, or 2000, a polyoxyethylenediamine with a molecular weight of around 600, 900, 2000, 4000 or 6000, a polyalkoxyethyleneamine with a molecular weight of around 600, 1000, 2000, a polyoxypropylenetriamine with a molecular weight of around 470, a long chain alkylamine with a mean molecular weight from 140 to 550, a (long chain alkyl)-aminopropylamine with a mean molecular weight from 280 to 360.
The ketone which is used for the blocking of the hydrazide function in the preparation of the product of formula (1) is prefarably acetone or butyl ketone.
The product of formula (1) is a polyamine which is modified by replacing an Nxe2x80x94H function by a N-hydrazone function. The number of hydrazone functions can be varied from 0.1-1 equimolar amount regarding to the NH-functions in the polyamine.
Furthermore the invention relates to the process for the preparation of the aqueous dispersion of a crosslinkable polyurethane containing hydrazone functions comprising:
dispersion of an isocyanate functional polyurethane prepolymer in water, optionally in the presence of neutralizing agents and/or surfactants
chain extension of the prepolymer with a hydrazone functional product containing at least two hydrogen functional amino groups, optionally in the presence of a surfactant, during or after the dispersion in water.
The isocyanate functional polyurethane prepolymer may be anionic, kationic of non-ionic and is prepared in a conventional way. EP 308115 presents an overview of suitable polyols and polyisocyanates for such polyurethane prepolymers.
The polyurethane dispersion can be made self-crosslinkable when the polyurethane contains both ketone and hydrazone functions. The invention therefore provides a process for the preparation of a selfcrosslinkable aqueous dispersion of a hydrazone and ketone functional polyurethane by using an isocyanate functional prepolymer containing ketone functions in the method described. The preparation of these types of prepolymers is described in U.S. Pat. No. 4,983,662.
The aqueous dispersions of crosslinkable polyurethane containing hydrazone functions obtained by the process of the invention may be used as coating compositions together with a crosslinking material. Accordingly the invention provides a process for the preparation of a crosslinked film or of a crosslinked coating onto a substrate comprising the steps of:
mixing the polyurethane dispersion containing hydrazone functions of the invention with a polyisocyanate, polyketone or polyepoxy functional material or aqueous dispersion thereof in which the ratio of functional groups is hydrazone: isocyanate or ketone is from 0.5 to 1.5 and in which the ratio of hydrazone: epoxy is from 0.3 to 1.2,
applying said mixture onto a substrate
evaporation of water and/or solvents
When an aqueous dispersion of a polyurethane containing both hydrazone and ketone functions is used a self crosslinking systems is obtained. Therefore the invention includes the preparation of a crosslinked film or of a crosslinked coating onto a substrate comprising:
applying the hydrazone and ketone functional polyurethane dispersion onto a substrate and crosslinking the same by
evaporation of water and/or solvents.
Another option is that products of formula (1) are not incorporated in a polyurethane structure, but are used as pure hydrazone functional products. Also in this case they can be employed as coating composition together with a cross-linking material. Accordingly, the invention provides the preparation of a crosslinked film or of a crosslinked coating onto a substrate comprising:
preparation of a mixture of a hydrazone functional material according to formula (1) in which Q is not H with a polyketone or poly-epoxy functional material, or with an aqueous dispersion of a, polyketone or polyepoxy functional material, and preferably with an aqueous dispersion of a polyisocyanate functional material in which the ratio of functional groups is hydrazone: isocyanate or ketone is from 0.5 to 1.5 and in which the ratio of hydrazone: epoxy is from 0.3 to 1.2,
applying the mixture onto a substrate
evaporation of water and/or solvents.
The hydrazone functional products or polyurethane dispersions offer, together with a crosslinker a coating material which has a comparable and sometimes even better performance and properties than those described in EP507407. Especially the crosslinking with a polyisocyanate functional material is very efficient.
The dispersions of hydrazone functional polyurethanes are better resistant to yellowing than those of the hydrazide functional polyuretanes described in U.S. Pat. No. 4,598,121. In combination with polyisocyanates as a crosslinker the dispersions of hydrazone functional polyurethanes have a longer pot-life than the dispersions of the hydrazide functional polyurethanes. The reason is that the protection of the hydrazide function by acetone is stable in water at pH greater than 7 and the isocyanate function can only react with the hydrazide function after the acetone has been released during the film formation. Moreover, the crosslinking with polyisocyanate-, polyketone- or polyepoxy-functional materials is a method which gives less rise to environmental problems than the crosslinking of hydrazide functional compounds with formaldehyde, as described in U.S. Pat. No. 4,598,121.
Many additional ingredients may also be present in the application and crosslinking stage, for example fillers, colorants, pigments, silicons, matting agents, flow agents, and the like. These additional ingredients can be used when there is no or little interference with the crosslinking function, or, if there is interference, it contributes to an optimal application result.
The hydrazone functional products and aqueous dispersions of polyurethanes in combination with the polyisocyanate, polyketone or polyepoxy functional material may be used in adhesives,sealants, printing ink and in coatings. They may be applied on any substrates, including leather or artificial leather, metals, wood, glass, plastics, paper, paper board, textile, non-woven, cloth, foam and the like by conventional methods, including spraying, flow-coating, roller-coating, brushing, dipping, spreading and the like.